Liquid-crystalline mixtures, in particular ferroelectric liquid-crystalline mixtures

ABSTRACT

The novel liquid-crystalline mixtures (in the case of ferroelectric behavior they additionally contain an optically active compound as dope) are based on at least two mixture components of the general formula (I) and at least one carboxylate of the general formula (II), (III) or (IV). ##STR1## The mixtures have favorable properties for use in electrooptical display elements, such as high contrast, low melting point and short switching times.

Ferroelectric liquid crystals have recently attracted interest as adisplay medium in electrooptical assemblies (for example Lagerwall etal., "Ferroelectric Liquid Crystals for Displays", SID Symposium,October Meeting 1985, San Diego, Calif. USA).

For practical use of ferroelectric liquid crystals in electroopticaldisplays, chiral, tilted, smectic phases, such as S_(c) phases, arerequired (R. B. Meyer, L. Liebert, L. Strzelecki and P. Keller, J.Physique 36, L-69 (1975)|, which are stable over a broad temperaturerange. This aim can be achieved by means of compounds which themselvesform such phases, for example S_(c) phases, or, however, by dopingcompounds which form non-chiral, tilted, smectic phases, with opticallyactive compounds (M. Brunet, C. Williams, Ann. Phys. 3, 237 (1978)|.

Furthermore, a unitary planar orientation of the liquid crystals isnecessary in order to achieve a high contrast ratio when ferroelectricliquid-crystal mixtures are used in electrooptical assemblies. It hasbeen shown that a unitary planar orientation in the S_(c) phase can beachieved if the phase sequence of the liquid-crystal mixture withdecreasing temperature is as follows:

Isotropic T nematic T smectic A T smectic C.

(for example K. Flatischler et al., Mol. Cryst. Liq. Cryst. 131, 21(1985); T. Matsumoto et al., p. 468-470, Proc. of the 6th Int. DisplayResearch Conf., Japan Display, 30 Sep. - 2 Oct. 1986, Tokyo, Japan; M.Murakami et al., ibid., p. 344-347).

For ferroelectric (chiral smectic) liquid-crystal mixtures, thecondition that the pitch of the helix must be large, i.e. greater than 5μm, in the S_(c) * phase and must be very large, i.e. greater than 10 μmor infinite, in the N* phase must additionally be fulfilled.

The optical switching time τ[μs] of ferroelectric liquid-crystalsystems, which should be as short as possible, depends on the rotationviscosity of the system γ[mPas], the spontaneous polarization P_(s)[nC/cm² ] and the electric field strength E[V/m], in accordance with theequation ##EQU1## Since the field strength E is determined by theelectrode separation in the electrooptical assembly and by the appliedvoltage, the ferroelectric display medium must have low viscosity andhigh spontaneous polarization in order that a short switching time isachieved.

Finally, besides thermal, chemical and photochemical stability, a smalloptical anisotropy Δn, preferably <0.13, and a small positive or,preferably, negative dielectric anisotropy Δε are required. (S. T.Lagerwall et al., "Ferroelectric Liquid Crystals for Displays" SIDSymposium, Oct. Meeting 1985, San Diego, Calif., USA).

It is only possible to satisfy all these demands using mixturescomprising several components. The basis (or matrix) used here ispreferably compounds which if possible themselves already have thedesired phase sequence I→N→S₁ →S_(c). Further components of the mixtureare often added to reduce the melting point and to broaden the S_(c) andusually also the N phase, to induce optical activity, for pitchcompensation and to match the optical and dielectric anisotropy;however, the rotation viscosity, for example, should if possible not beincreased at the same time.

It is known that certain derivatives of phenylpyrimidine, in particular5-alkyl-2(4-alkoxyphenyl)pyrimidines, are capable of forming S_(c),S_(A) and N phases (D. Demus and H. Zaschke, "Flussige Kristalle inTabellen", [Liquid Crystals in Tables], VEB Deutscher Verlag furGrundstoffindustrie, Leipzig 1974, pp. 260-261) and, in addition, can beconverted into ferroelectric liquid-crystal mixtures by addition ofoptically active dopes [L. M. Blinov et al., Sow. Phys. lisp. 27 (7),492 (1984); L. A. Beresnew et al., Ferroelectrics, 59 (321]/1 (1984),presented at the 5th Conference of Soc. Countries on Liquid Crystals,Odessa, USSR, Oct. 1983; DE-A-3,515,347, EP-A 0,206,228 and EP-A0,225,195].

It is furthermore known that lower melting points and a broadening ofthe liquid-crystalline phases desired can be achieved by mixing severalliquid-crystalline compounds [D. Demus et al., Mol. Cryst. Liq. Cryst.25, 215 (1974), J. W. Goodby, Ferroelectrics 49, 275 (1983)], and thatthe melting-point depression is the more pronounced the more the mixturecomponents also differ structurally. (J. S. Dave et al., J. Chem. Soc.1955, 4305). It was thus to be presumed that particularly low mixturemelting points, and thus low phase-transition points of the S_(c) phase,are obtained when compounds are mixed which, on the one hand, aresignificantly different structurally, but, on the other hand, aresufficiently similar to be readily miscible.

DE-C 2,257,588 discloses a 5-butoxy-2(4-pentoxyphenyl)-pyrimidine, butthis only forms a nematic phase.

In DE 3,831,226.3, it has been found that, as the base component,compounds of the type 5-alkoxy-2(4-alkoxyphenyl)pyrimidines of thegeneral formula (I) ##STR2## in which C_(n) H_(2n+1) and C_(x) H_(2x+1)are straight-chain alkyl radicals in which n is an integer from 6 to 14and x is an integer f rom 2 to 14, are particularly highly suitable[lacuna] the matrix (base) for liquid-crystalline mixtures, inparticular ferroelectric liquid-crystalline mixtures; of them, at leasttwo different compounds a' and a" are present in the mixture. For themost part, they exhibit the desired phase sequence ITNTS_(A) TS_(c) withdecreasing temperature. Compared with the known5-alkyl-2-(4-alkoxyphenyl)pyrimidines, they have considerably broaderS_(c) phases and higher transition temperatures of the S_(c) TS_(A)transition. In addition, the S_(c) phase is even formed in the case of alower number of carbon atoms in the alkyl(oxy) chains, for example evenwhen n =8 and x 2 or n =6 and x 6. Compared with the known5-alkyl-2-(4-alkoxyphenyl)pyrimidines, this gives a greater choice ofhomologous, readily miscible compounds having S_(c) phases for preparingthe mixture. Due to their higher phase-transition temperatures S_(A) toS_(c) (for example 70° to 90° C.) and their broader S_(c) phases, theyalso make it possible to prepare mixtures having very broad S_(c) phases(for example up to about 68° C.) and high transition temperatures S_(A)/S_(c).

The mixtures of DE 3,831,226.3 of compounds of the formula (I) arealready very highly suitable as a matrix for ferroelectricliquid-crystalline mixtures. However, they also have a large opticalanisotropy (Δn) and a large positive dielectric anisotropy (Δε). Inaddition, the nematic phase, depending on the chain length n or x of thesubstituents, is still narrow.

By adding at least one compound (cyclohexanecarboxylate) of the generalformula (II), i.e. a compound of type b, ##STR3## the parameters [Δn]and [Δε] can simultaneously be optimized and the width of the nematicphase can be matched to practical requirements.

In the general formula (II):

R¹ is an alkyl chain having 10 to 16 carbon atoms or an alkoxy chainhaving 8 to 14 carbon atoms, and R² is an alkyl chain having 2 to 9carbon atoms. These compounds are described in DE-A 3,731,639. Theydevelop their favorable action even when added in amounts from about 2mol %, relative to the mixture of the components of type a and b. Whenadded in amounts from about 10 and up to about 40 mol %, the nematicphase is broadened by about 8 to 18° C. and the smectic phase by about6° to 15° C. In addition, it is very advantageous that the addition onlyinsignificantly increases the rotation viscosity of the base mixtureaccording to the invention, which is already very low per se.

The melting point and the lower phase-transition temperature of theS_(c) phase of the mixtures can be reduced, if they are too high, byadding at least one compound (alkenyloxyphenylpyrimidine derivative) ofthe general formula (III), i.e. a compound of type c, to mixtures ofcompounds of type a alone or additionally containing compounds of typeb: ##STR4##

In the general formula (III), R³ is a branched or straight-chain alkylradical having 7 to 16 carbon atoms or a straight-chain or branchedalkoxy radical having 6 to 14 carbon atoms, and y is an integer f rom 4to 14. These compounds are described in DE-A 3,731,638. Added amounts offrom about 10 to 35 mol %, relative to the overall mixture, reduce thelower temperature limit of the S_(c) phase by up to 5° C. The otherfavorable physical properties of the mixtures are generally not impairedby this addition.

To reduce the melting point and the lower temperature limit of the S_(c)phase, it is also possible to add, together with or in place ofcompounds of type c, other compounds of the formula (IV), i.e. of typed, ##STR5## where m and p, independently of one another, are integersfrom 6 to 14. When amounts of from about 10 to 25 mol % of the compoundof type d or mixtures with compounds of type c, relative to the overallmixture, are added to mixtures of compounds of type a or a and b, thelower temperature limit of the S_(c) phase is reduced by up to 10° C.

Some of these components, and also certain mixtures, are already knownfrom the prior art. However, since the development, in particular offerroelectric liquid-crystal mixtures, can in no way be regarded ascomplete, the manufacturers of display elements are interested in a verywide variety of mixtures. A further reason for this, amongst others, isthat conclusions on the quality of the liquid-crystalline mixtures toocan only be made by considering the interactions of theliquid-crystalline mixtures with the individual assemblies of thedisplay elements or of the cells (for example the orientation layer).

The object of the present invention is therefore to provide compositionsof suitable components for liquid-crystalline base mixtures, but alsofor ferroelectric liquid-crystalline mixtures, which satisfy as many aspossible of the abovementioned criteria, have, in particular, a lowmelting point and furthermore result in improved contrast in a display.

The invention relates to the liquid-crystalline base mixturescharacterized below, which contain, on the one hand, compounds A from DE3,831,226.3, but, on the other hand, also contain at least onecarboxylate C of the general formula (V), (VI), (VII), (VIII) or (IX) asa mixture component.

The invention furthermore relates to liquid-crystalline mixtures havingfavorable values for the optical anisotropy and the dielectricanisotropy and having a broad nematic phase which is sufficient forpractical use.

A preferred subject-matter of the invention are likewise ferroelectricliquid-crystalline mixtures having a short switching time, the phasesequence I→N*→S_(A), S_(c) *, the pitch in the nematic phase being >15μm and in the S_(c) * phase being >5 μm, and having a negative Δε andΔn<0.15, and which contain a chiral dope C in addition to the componentsA and the carboxylates B.

The novel invention relates to liquid-crystalline mixtures containing,as component A, at least two 5-alkoxy-2-(alkoxyphenyl)pyrimidines of thegeneral formula (I) ##STR6## in which the alkyl radicals arestraight-chain alkyl radicals in which n is an integer from 6 to 14 andx is an integer from 2 to 14, and, if appropriate, one or morecyclohexanecarboxylates of the general formula (II) ##STR7## in which R¹is an alkyl radical having 10 to 16 carbon atoms or an alkoxy radicalhaving 8 to 14 carbon atoms, and R² is an alkyl radical having 2 to 9carbon atoms, and, if appropriate, one or morealkenyloxyphenylpyrimidine derivatives of the general formula (III)##STR8## in which R³ is an alkyl chain having 7 to 16 carbon atoms or analkoxy chain having 6 to 14 carbon atoms, and y is an integer from 4 to14, and, if appropriate, one or more alkylpyrimidinealkoxyphenylderivatives of the general formula (IV) ##STR9## in which m and p,independently of one another, are integers from 6 to 14, and ifappropriate as component B (for ferroelectric liquid-crystal mixtures),at least one optically active compound from the group

a) optically active esters made from α-chlorocarboxylic acids andmesogenic phenols,

b) optically active esters of N-acylproline,

c) optically active esters of 1,3-dioxolane-4-carboxylic acids, and

d) optically active esters of oxirane-2-carboxylic acids,

wherein the liquid-crystal mixtures furthermore contain at least onecarboxylate of the general formula (V), (VI), (VII), (VIII) or (IX) as amixture component: ##STR10## where k may be an integer from 6 to 14, and

l may be an integer from 2 to 14, ##STR11## where m may be an integerfrom 5 to 14, and

l may be an integer from 2 to 14, ##STR12## where p may be an integerfrom 7 to 14, and

r may be an integer from 4 to 14, ##STR13## where s may be an integerfrom 6 to 14, and

t may e an integer from 6 to 14, ##STR14## where R² is straight-chain orbranched (C₁ -C₁₂)alkyl or alkenyl, it being possible for one or twonon-adjacent CH₂ groups to be replaced by O and/or S atoms,

Y is F, Cl, Br, CN or CF₃ and

R¹ is branched (C₃ -C₉)alkyl, benzyl or phenyl.

Preferred liquid-crystal mixtures are those which contain, as componentC, at least one 4-(5-alkylpyrimidin-2-yl)phenyl carboxylate of thegeneral formula (V) ##STR15## where k may be an integer from 6 to 14,and

l may be an integer from 4 to 14.

Likewise preferred mixtures are those which contain, as component C, atleast one 4-(5-alkoxypyrimidin-2-yl)-phenyl carboxylate of the generalstructure (VI) ##STR16## where m may be an integer from 6 to 14, and

l may be an integer from 4 to 14.

Preferred liquid-crystalline mixtures are also those which contain, ascomponent C, at least one benzoate of the general structure (VII)##STR17## where p may be an integer from 7 to 14, and

r may be an integer from 4 to 14.

Likewise preferred mixtures are those which contain, as component C, atleast one benzoate of the general structure (VIII) ##STR18## where s maybe an integer from 6 to 14, and

t may be an integer from 6 to 14.

In addition, preferred mixtures are those which contain, at least as onecomponent, a racemate of the structure (IX) ##STR19## where R² isstraight-chain or branched (C₁ -C₁₂)alkyl or alkenyl, it being possiblefor one or two non-adjacent CH₂ groups to be replaced by O and/or Satoms,

Y is F, Cl or CF₃ and

R¹ is branched (C₃ -C₉)alkyl, benzyl or phenyl.

Particularly preferred liquid-crystal mixtures are those which contain,as component C, the racemate of at least one α-halocarboxylate of thestructure (IXa) ##STR20## where R² may be an alkyl radical or an alkenylradical having 1 to 12 carbon atoms whose CH₂ group bonded to thearomatic rings may also be replaced by O and/or [lacuna], and y is F orCl.

Very particularly suitable here are the α-halocarboxylic acids of thestructures (IXb) and (IXc) ##STR21##

It has now been found that the FLC mixtures according to the inventionwhich contain, as component C, 2 to 25 mol-%, in particular 10 to 25mol-%, of mixtures of individual compounds of the general structures (V)and/or (VI) and/or (VIII) and/or (VIII) and/or (IX) have an upperphase-transition point which is up to 10° C. lower than thecorresponding mixtures without the components according to theinvention.

Furthermore, the contrast is increased by up to 10% and, in some cases,the switching times are also reduced by the addition.

Through the specific addition of the abovementioned compounds of theformulae (V to IX), the physical parameters [Δn] and [Δε] of the mixturecan be optimized, i.e. matched to the particular specific requirementsof the display element.

Surprisingly, it has furthermore been observed that liquid-crystallinemixtures which contain 2 to 15 mol-% of racemic mixtures of one or moreesters of the general structure (IX) have a melting point which is lowerby up to 5° C. and an upper S_(c) phase transition point which is lowerby up to 10° C. than the corresponding base mixture without this (these)component(s). The contrast of the mixtures according to the invention isup to 20% higher than that of the base mixtures. Doping with a racemicmixture of the α-chlorocarboxylate (IXb) ##STR22## has provenparticularly suitable here.

The liquid-crystal mixtures according to the invention are suitable bothas base mixtures and, when chiral dopes B are added, as novelferroelectric liquid-crystal mixtures which are particularly suitablefor practical use in electrooptical switching and display elements.

As already proposed (DE 3,831,226.3), the following dopes have provenparticularly suitable for the purpose of the invention:

a) optically active esters made from α-chlorocarboxylic acids andmesogenic phenols as in DE-A 3,703,228, and, in particular, theoptically active ester of the formula below ##STR23## where r=4 to 8 b)optically active esters of N-acylproline as in DE-A 3,644,522

c) optically active esters of 1,3-dioxolane-4-carboxylic acids as inDE-A 3,713,273

d) optically active esters of oxirane-2-carboxylic acids as in DE-A3,718,174.

These dopes frequently induce ferroelectric behavior even when added insmall amounts from 0.5 and in particular from 3 mol-%, relative to theoverall mixture. The upper limit for the amount added is 30 mol-%.

The addition of these dopes generally further reduces the melting pointand the lower temperature limit of the S_(c) * phase. In addition,depending on the nature and amount of the dope, the upper temperaturelimit of the S_(c) * phase may be reduced or increased and the width ofthe N* phase may be changed, which should be taken into account whenpreparing the matrix mixture. As is known, the optically active dopeinduces helical twisting in the S_(c) phase and in the N phase, whichconverts these phases into the S_(c) * phase and N* phase respectively.The pitch and the rotation sense of these helices in the N* phase dependon the nature and amount of the optically active dope. They mustfrequently be compensated by adding a further dope which induces a helixwith the opposed rotation sense. In general, a ferroelectricliquid-crystal mixture should have a helix with a pitch of greater than10 μm in the N* phase and of at least 5 μm in the S_(c) * phase. Thecompounds used for compensation of the pitch should preferably notinduce in the mixture any spontaneous polarization or should only do soto a negligibly small extent, or should induce a spontaneouspolarization with the same sign as the optically active dope.

If pitch compensation is necessary, the following combinations areparticularly suitable in the mixtures according to the invention:

a₁) Optically active α-chlorocarboxylates induce a negative P_(s) and ahelix with negative rotation sense. For compensation, derivatives ofcitronellol of the general formula (N) are advantageously added##STR24##

In this formula, q is an integer from 6 to 16, A is an oxygen atom or achemical bond, and B is a pyrimidine-2,5-diyl ring, it being possiblefor the phenylene ring bonded thereto to be in the 2- or 5-position. Forpitch compensation, up to 4 mol %, relative to the overall mixture, orone mole of citronellol derivative are required per 7 to 13 moles ofα-chlorocarboxylate.

b₁) Optically active N-acylproline esters having the S configuration atthe asymmetric carbon atom which induce a positive P_(s) and a negativepitch in the mixtures according to the invention can be compensated byadding optically active oxirane-2-carboxylates as in DE-A 3,718,174having a positive P_(s) and, in particular, a 2-R-3-R configuration. Theamount to be added is up to 6 mol %, relative to the overall mixture or1 mole of oxirane-2-carboxylate per 5 to 9 moles of N-acylproline ester.

c₁) The compensation in the case of optically active1,3-dioxolane-4-carboxylates having the R configuration, which likewiseinduce a positive P_(s) and a negative pitch, is likewise expedientlycarried out as described under b,) using oxirane-2-carboxylates.

The use of oxirane-2-carboxylates as described in b₁) and c₁) has theadditional advantage that these compounds themselves induce a very highpositive P_(s) and thus increase the P_(s) of the overall mixture andshorten the switching time.

The values of spontaneous polarization P_(s) [nC /CM² ], the contrast K,the optical switching time τ [μs], the dielectric anisotropy Δε and theoptical anisotropy Δn were determined for the ready-to-use ferroelectricliquid-crystal mixtures (all measurements at 25° C.).

The P_(s) values are measured by the method of H. Diamant et al. (Rev.Sci. Instr., 28, 30, 1957), where measurement cells with an electrodeseparation of 2 μm and ground polyamide as the orientation layer areused.

To determine τ and K, the measurement cell is clamped between crossedanalyzer and polarizer on the rotary stage of a polarizing microscope.

To determine the contrast (K), the measurement cell is positioned, byrotation, so that a photodiode indicates minimum light transmission(dark state). The microscope illumination is adjusted so that thephotodiode indicates the same light intensity for all cells. After aswitching operation, the light intensity changes (bright state), and thecontrast is calculated from the ratio of the light intensities in thesestates.

The switching time τ is determined using a photodiode by measuring thetime taken for the light signal to increase from a signal height of 10to 90 %. The switching voltage comprises rectangular pulses and is +10V/μm.

The phase-transition temperatures are determined using a polarizingmicroscope from the texture changes on heating. By contrast, the meltingpoint is determined using a DSC instrument. The phase-transitiontemperatures between the phases

    ______________________________________                                        Nematic              (N or N*)                                                Smectic-C            (S.sub.c or S.sub.c *)                                   Smectic-A            (S.sub.A or S.sub.A *)                                   Crystalline          (X)                                                      ______________________________________                                    

are given in °C., and the values are between the phase symbols in thephase sequence.

Example 1

a) a liquid-crystalline mixture comprising the following eightcomponents

    ______________________________________                                        5-octoxy-2-(4-hexoxyphenyl)pyrimidine                                                                     14 mol-%                                          5-octoxy-2-(4-octoxyphenyl)pyrimidine                                                                      5 mol-%                                          5-octoxy-2-(4-butoxyphenyl)pyrimidine                                                                     15 mol-%                                          5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                      8 mol-%                                          5-octoxy-2-(4-dodecoxyphenyl)pyrimidine                                                                    8 mol-%                                          5-octyl-2-(4-decoxyphenyl)pyrimidine                                                                      13 mol-%                                          [4-(5-decylpyrimidin-2-yl)]phenyl trans-                                                                  17 mol-%                                          4-pentylcyclohexanecarboxylate                                                 ##STR25##                                                                    [4-(5'-octylpyrimidin-2-yl)]phenyl                                                                        20 mol-%                                          heptanoate (= compound of the general formula V)                              exhibits the following liquid-crystalline phase                               ranges:                                                                       X -5 S.sub.c 71 S.sub.A 78 N 93 I                                             ______________________________________                                    

b) The following mixture, which is claimed in DE 3,831,226.3 and differsfrom the mixture 1a only in that it contains no component of the generalformula V, is suitable for comparing the physical properties. Thismixture comprising seven components has the following liquid-crystallinephase ranges:

    ______________________________________                                        X -1       S.sub.c 81                                                                              S.sub.A 87                                                                              N 103   I                                      ______________________________________                                    

Compared with the mixture 1b, the mixture 1a according to the inventionhas a melting point (X) which is lower by 4° C. and an upper S_(c)phase-range limiting temperature which is lower by 10° C.

Example 2

a) A liquid-crystalline mixture comprising the following eightcomponents

    ______________________________________                                        5-octoxy-2-(4-hexoxyphenyl)pyrimidine                                                                     16 mol-%                                          5-octoxy-2-(4-octoxyphenyl)pyrimidine                                                                      6 mol-%                                          5-octoxy-2-(4-butoxyphenyl)pyrimidine                                                                     15 mol-%                                          5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                      8 mol-%                                          5-octoxy-2-(4-dodecoxyphenyl)pyrimidine                                                                    8 mol-%                                          5-octyl-2-(4-dodecoxyphenyl)pyrimidine                                                                    13 mol-%                                          [4-(5-decylpyrimidin-2-yl)]phenyl trans-                                                                  19 mol-%                                          4-pentylcyclohexanecarboxylate                                                 ##STR26##                                                                    [4-(5'-octylpyrimidin-2-yl)]phenyl                                                                        15 mol-%                                          heptanoate (= compound of the general formula V)                              exhibits the following liquid-crystalline phase                               ranges:                                                                       X -5 S.sub.c 75 S.sub.A 80 N 87 I                                             ______________________________________                                    

b) The following mixture, which is claimed in DE 3,831,226.3 and differsfrom the mixture 2a only in that it contains no component of the generalformula V, is suitable for comparing the physical properties.

This mixture comprising seven components has the followingliquid-crystalline phase ranges:

    ______________________________________                                        X -2       S.sub.c 81                                                                              S.sub.A 86                                                                              N 104   I                                      ______________________________________                                    

Compared with the mixture 2b, the mixture 2a according to the inventionhas a melting point which is lower by 3° C. and an upper S_(c)phase-range limiting temperature which is lower by 6° C.

Example 3

a) A liquid-crystalline mixture comprising the following eightcomponents

    ______________________________________                                        5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                     15 mol-%                                          5-octoxy-2-(4-octoxyphenyl)pyrimidine                                                                      6 mol-%                                          5-octoxy-2-(4-butoxyphenyl)pyrimidine                                                                     15 mol-%                                          5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                      8 mol-%                                          5-octoxy-2-(4-dodecoxyphenyl)pyrimidine                                                                    8 mol-%                                          5-octyl-2-(4-dodecoxyphenyl)pyrimidine                                                                    14 mol-%                                          [4-(5-decylpyrimidin-2-yl)]phenyl trans-                                                                  14 mol-%                                          4-pentylcyclohexanecarboxylate                                                 ##STR27##                                                                    [4-(5-octylpyrimidin-2-yl)]phenyl                                                                         20 mol-%                                          heptanoate (= compound of the general formula V)                              exhibits the following liquid-crystalline phase                               ranges:                                                                       X -3.5 S.sub.C 69 S.sub.A 80 N 92 I                                           ______________________________________                                    

b) The following mixture, which is claimed in DE 3,831,226.3 and differsfrom the mixture 3a only in that it contains no component of the generalformula V, is suitable for comparing the physical properties.

This mixture comprising seven components has the followingliquid-crystalline phase ranges:

    ______________________________________                                        X 0        S.sub.c 80                                                                              S.sub.A 88                                                                              N 102   I                                      ______________________________________                                    

In comparison, the mixture 3a according to the invention has a meltingpoint which is 3.5° C. lower and an upper S_(c) phase-range limitingtemperature which is 11° C. lower.

Example 4

a) A liquid-crystalline mixture comprising the following eightcomponents

    ______________________________________                                        5-octoxy-2-(4-hexoxyphenyl)pyrimidine                                                                     16 mol-%                                          5-octoxy-2-(4-octoxyphenyl)pyrimidine                                                                      6 mol-%                                          5-octoxy-2-(4-butoxyphenyl)pyrimidine                                                                     16 mol-%                                          5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                      8 mol-%                                          5-octoxy-2-(4-dodecoxyphenyl)pyrimidine                                                                    8 mol-%                                          5-octyl-2-(4-dodecoxyphenyl)pyrimidine                                                                    14 mol-%                                          [4-(5-decylpyrimidin-2-yl)]phenyl trans-                                                                  22 mol-%                                          4-pentylcyclohexanecarboxylate                                                 ##STR28##                                                                    4-(5-decylpyrimidin-2-yl)phenyl                                                                           10 mol-%                                          dodecanoate (= compound of the general formula V)                             exhibits the following liquid-crystalline phase                               ranges:                                                                       X -5.5 S.sub.c 79 S.sub.A 84 N 94 I                                           ______________________________________                                    

b) The following mixture, which is claimed in DE 3,831,226.3 and differsfrom the mixture 4a only in that it contains no component of the generalformula V, is suitable for comparing the physical properties.

This mixture comprising seven components has the followingliquid-crystalline phase ranges:

    ______________________________________                                        X -2       S.sub.c 81                                                                              S.sub.A 86                                                                              N 104   I                                      ______________________________________                                    

In comparison, the mixture 4a according to the invention has a meltingpoint which is 3.5° C. lower and an upper S_(c) phase-range limitingtemperature which is 2° C. lower.

Example 5

a) A liquid-crystalline mixture comprising the following nine components

    ______________________________________                                        5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                    7.5 mol-%                                          5-octoxy-2-(4-octoxyphenyl)pyrimidine                                                                    4 mol-%                                            5-octoxy-2-(4-hexoxyphenyl)pyrimidine                                                                    9 mol-%                                            5-octoxy-2-(4-butoxyphenyl)pyrimidine                                                                    9.5 mol-%                                          5-octyl-2-(4-decoxyphenyl)pyrimidine                                                                     9.5 mol-%                                          5-octyl-2-(4-hexoxyphenyl)pyrimidine                                                                     14 mol-%                                           5-octyl-2-(4-octoxyphenyl)pyrimidine                                                                     13 mol-%                                           [4-(5-decylpyrimidin-2-yl)]phenyl trans-                                                                 28.5 mol-%                                         4-pentylcyclohexanecarboxylate                                                 ##STR29##                                                                    4-(5-octylpyrimidin-2-yl)phenyl                                                                          5 mol-%                                            decanoate (= compound of the general formula V)                               exhibits the following liquid-crystalline phase                               ranges:                                                                       X -3 S.sub.c 68 N 94 I                                                        ______________________________________                                    

b) A comparable mixture which differs from the mixture 5a only in thatit contains no component of the general formula V has the followingliquid-crystalline phase ranges:

    ______________________________________                                        X -2         S.sub.c 80 N 99       I                                          The addition of the component of the general                                  formula V has thus resulted in a reduction of the                             melting point by 1° C.                                                 ______________________________________                                    

Example 6

a) A liquid-crystalline mixture comprising the following eightcomponents

    ______________________________________                                        5-octoxy-2-(4-hexoxyphenyl)pyrimidine                                                                     16 mol-%                                          5-octoxy-2-(4-octoxyphenyl)pyrimidine                                                                      6 mol-%                                          5-octoxy-2-(4-butoxyphenyl)pyrimidine                                                                     15 mol-%                                          5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                      8 mol-%                                          5-octoxy-2-(4-dodecoxyphenyl)pyrimidine                                                                    8 mol-%                                          5-octyl-2-(4-dodecoxyphenyl)pyrimidine                                                                    13 mol-%                                          [4-(5-decylpyrimidin-2-yl)]phenyl trans-                                                                  19 mol-%                                          4-pentylcyclohexanecarboxylate                                                 ##STR30##                                                                    4-(5-octylpyrimidin-2-yl)phenyl                                                                           15 mol-%                                          undecanoate (= compound of the general formula V)                             exhibits the following liquid-crystalline phase                               ranges:                                                                       X -5 S.sub.c 75 S.sub.A 80 N 87 I                                             ______________________________________                                    

b) The following mixture, which is claimed in DE 3,831,226.3 and differsfrom the mixture 6a only in that it contains no component of the generalformula V, is suitable for comparing the physical properties.

This mixture comprising seven components has the followingliquid-crystalline phase ranges:

    ______________________________________                                        X -2       S.sub.c 81                                                                              S.sub.A 86                                                                              N 104   I                                      ______________________________________                                    

In comparison, the mixture 6a according to the invention has a meltingpoint which is 3° C. lower and an upper S_(c) phase-range limitingtemperature which is 6° C. lower.

Example 7

The following ferroelectric mixtures comprising the following components##STR31##

In Table 1 below, the property features of the examples are reproducedin a summarized form.

                                      TABLE 1                                     __________________________________________________________________________    Concentration     Switching time                                                                        Polarization                                                                         Phase-transition temperatures                     X   Y   Contrast                                                                           τ   P.sub.s                                                                              X→S.sub.c *                                                                S.sub.c *→S.sub.A *                                                         S.sub.A *→N*                                                                N*→I                    Example                                                                            Mol %                                                                             Mol %                                                                             K    μs   nC · cm.sup.-2                                                              °C.                                                                        °C.                                                                         °C.                                                                         °C.                     __________________________________________________________________________    7a   94   0  6.4  75      -10    -1  80   85   99                             7b   79  15  4.6  65      -10.5  -8  73   79   92                             __________________________________________________________________________     The results in columns 5 and 7 show very clearly the improvement in the       switching time by 10 μs and in the melting point by 7° C. with      increased concentration of the compound of the general formula V.        

Example 8

a) A ferroelectric mixture comprising the following components

    __________________________________________________________________________    mixture from Example 2a                               94 mol-%                 ##STR32##                                             6 mol-%                4-[2-((S)-7-methylnonoxy)pyrimidin-5-yl]phenyl (2S,3S)-2-chloro-3-methylpe    ntanoate                                                                      exhibits the following LC ranges                                              X -9 S.sub.c * 72 S.sub.A * 78 N* 93 I                                        __________________________________________________________________________

At 25° C., this mixture has a polarization of -9 nC.cm⁻² and, in aswitching field of 10 Vμm⁻¹, a switching time of 55 μs.

For comparison, a mixture based on the base mixture of Example 2b issuitable.

b) The ferroelectric mixture comprising

    __________________________________________________________________________    mixture from Example 2b                                94 mol-%                ##STR33##                                              6 mol-%               4-[2-((S)-7-methylnonoxy)pyrimidin-5-yl]phenyl (2S,3S)-2-chloro-3-methylpe    ntanoate                                                                      exhibits the following liquid-crystalline phase ranges:                       X -7 S.sub.c 79 S.sub.A 84 N 99 I                                             __________________________________________________________________________

At 25° C., this mixture has a spontaneous polarization of -9 nC cm⁻², acontrast of 5.6 and, in a switching field of 10 Vμm⁻¹, a switching timeof 60 μs.

Compared with the mixture 9a, this mixture contains no component of thegeneral formula VI and, in comparison, has a melting point which is 2°C. higher and a switching time which is 5 μs longer.

Example 9

A ferroelectric mixture comprising the following four components

    __________________________________________________________________________    mixture from Example 2b                                   64 mol-%             ##STR34##                                                15 mol-%            4-(decylpyrimidin-2-yl)phenyl nonanoate (compound of the general formula      V)                                                                             ##STR35##                                                15 mol-%            4-(5-decylpyrimidin-2-yl)phenyl undecanoate (compound of the general          formula V)                                                                     ##STR36##                                                6 mol-%             4-[2-(S)-7-methylnonoxypyrimidin-5-yl] phenyl(2S,3S)-2-chloro-3-methylpent    anoate                                                                        exhibits the following liquid-crystalline phase ranges:                       X -11 S.sub. c * 75 S.sub.A * 90 N* 85 I                                      __________________________________________________________________________

At 25° C., this mixture has a polarization of -8.7 nC cm⁻² a contrast of6.6 [lacuna] in a switching field of 10 Vμm⁻¹, a switching time of 50μs.

Compared with the mixture 8b, which differs from the mixture 9 only inthat it contains no compound of the general formula V, the mixture 9 hasa contrast which is higher by 18%, a melting point which is lower by 4°C. and a switching time which is shorter by 10 μs.

Example 10

A ferroelectric mixture comprising the following components

    __________________________________________________________________________    mixture from Example 2a          89 mol-%                                      ##STR37##                       5 mol-%                                      4-(2-nonoxypyrimidin-5-yl)phenyl                                              2-chloro-4-methyl-pentanoate (racemate)                                       (compound of the general formula IX)                                           ##STR38##                       6 mol-%                                      4-(2-nonoxypyrimidin-5-yl)phenyl                                              (2S,3S)-2-chloro-3-methylpentanoate                                           has the phase sequence                                                        X -9 S.sub.c * 72 S.sub.A * 77 N* 88 I                                        __________________________________________________________________________

At 25° C., this mixture has a polarization of -8.7 nC cm⁻², a contrastof 8 and, in a switching field of 10 Vμm⁻¹, a switching time of 60 μs.

Compared with the ferroelectric mixture of Example 7b, the contrast is45% higher. The addition of the racemic α-chlorocarboxylate thus resultsin an increase in the contrast and a reduction in the melting point by1° C.

Example 11

a) A ferroelectric mixture comprising the following components

    __________________________________________________________________________    mixture from Example 3a          88 mol-%                                      ##STR39##                       12 mol-%                                     4-(2-nonoxypyrimidin-5-yl)phenyl                                              (2S,3S)-2-chloro-3-methylpentanoate                                           has the phase sequence                                                        X -7 S.sub.c * 68 S.sub.A * 74 N* 85 I                                        __________________________________________________________________________

At 25° C., this mixture has a polarization of -20n C cm⁻² [sic], acontrast of 8.2 and, in a switching field of 10Vμm⁻¹, a switching timeof 18 μs.

b) A ferroelectric mixture comprising

    __________________________________________________________________________    mixture from Example 3a          85 mol-%                                      ##STR40##                       12 mol-%                                     4-(2-nonoxypyrimidin-5-yl)phenyl                                              (2S,3S)-2-chloro-3-methylpentanoate                                            ##STR41##                        3 mol-%                                     4-(2-nonoxypyrimidin-5-yl)phenyl 2-chloro-4-methyl-                           pentanoate (racemate)                                                         __________________________________________________________________________

has the phase sequence

    ______________________________________                                        X -8      S.sub.c * 68                                                                            S.sub.A * 75                                                                            N* 85.5  I                                      ______________________________________                                    

At 25° C., this mixture has a polarization of -20 nC cm⁻², a contrast of8.9 and, in a switching field of 10 Vμm⁻¹, a switching time of 17 μs.Compared with the ferroelectric mixture in Example 11a, the contrast ishigher. The addition of the racemic α-chlorocarboxylate thus results inan increase in the contrast by 9%.

Examples 12a-c

The following ferroelectric mixtures comprising the following components##STR42##

In Table 2 below, the property features (at 25° C.) of the examples havebeen summarized.

                                      TABLE 2                                     __________________________________________________________________________    Concentration     Switching time                                                                        Polarization                                                                         Phase-transition temperatures                     X   Y   Contrast                                                                           τ   P.sub.s                                                                              X→S.sub.c *                                                                S.sub.c *→S.sub.A *                                                         S.sub.A *→N*                                                                N*→I                    Example                                                                            Mol %                                                                             Mol %                                                                             K    μs   nC · cm.sup.-2                                                              °C.                                                                        °C.                                                                         °C.                                                                         °C.                     __________________________________________________________________________    12a  94  0   4.5  43      -9.8   -6  70   77   86                             12b  91  3   4.6  44      -9.6   -5.3                                                                              69   77   85                             12c  88  6   5.8  45      -9.2   -5.5                                                                              68   76   84                             __________________________________________________________________________     The results in columns 3 and 4 show very clearly the improvement in the       contrast with the increase in the concentration of the racemic mixture of     the chlorocarboxylate (Y).                                               

Examples 13a-c

The following ferroelectric mixtures comprise the following components##STR43##

In Table 3 below, the property features (at 25° C.) of the examples arereproduced in a summarized form.

                                      TABLE 3                                     __________________________________________________________________________    Concentration     Switching time                                                                        Polarization                                                                         Phase-transition temperatures                     X   Y   Contrast                                                                           τ   P.sub.s                                                                              X→S.sub.c *                                                                S.sub.c *→S.sub.A *                                                         S.sub.A *→N*                                                                N*→I                    Example                                                                            Mol %                                                                             Mol %                                                                             K    μs   nC · cm.sup.-2                                                              °C.                                                                        °C.                                                                         °C.                                                                         °C.                     __________________________________________________________________________    13a  94  0   4.5  43      -9.8   -6  71   78   89                             13b  91  3   4.7  52      -9.5   -6  71   77   88                             13c  88  6   6.2  53      -9.2   -6  70   77   86                             __________________________________________________________________________     The results in columns 3 and 4 show very clearly the improvement in the       contrast with the increase in the concentration of the racemic mixture of     the chlorocarboxylate (Y).                                               

Example 14

a) A liquid-crystalline mixture comprising the following five components

    ______________________________________                                        5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                     21 mol-%                                          5-octoxy-2-(4-octoxyphenyl)pyrimidine                                                                     12 mol-%                                          5-octoxy-2-(4-hexoxyphenyl)pyrimidine                                                                     25 mol-%                                          5-octoxy-2-(4-butoxyphenyl)pyrimidine                                                                     27 mol-%                                           ##STR44##                  15 mol-%                                          4'-hexoxyphenyl 4-octoxybenzoate                                              (= compound of the general formula VII)                                       has the following liquid-crystalline phase ranges:                            X 8 S.sub.c 72.3 S.sub.A 86.1 N 93 I                                          ______________________________________                                    

b) A mixture without addition of 15 mol-% of the compound of the generalformula (VII) has, by contrast, the following liquid-crystalline phaseranges:

    ______________________________________                                        X 13      S.sub.c 81.5                                                                             S.sub.A 95.5                                                                            N 98    I                                      ______________________________________                                    

Compared with mixture 14b, the mixture 14a according to the inventionadditionally contains a component of the general formula (VII) and has,in comparison, a melting point which is lower by 5° C. and an upperS_(c) phase-range limiting temperature which is lower by 9.5° C.

Example 15

A liquid-crystalline mixture comprising the following five components

    ______________________________________                                        mixture from Example 14b    80 mol-%                                           ##STR45##                  20 mol-%                                          4-hexoxyphenyl 4'-decoxybenzoate                                              (compound of the general formula VII)                                         has the following liquid-crystalline phase ranges:                            X 9 S.sub.c 72.6 S.sub.A 87.3 N 92.5 I                                        ______________________________________                                    

Compared with the mixture 14b, the mixture additionally contains acomponent of the general formula (VII) and has, in comparison, a meltingpoint which is lower by 4° C. and an S_(c) phase-range limitingtemperature which is lower by 9° C.

Example 16

A liquid-crystalline mixture comprising the following six components

    ______________________________________                                        5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                     20 mol-%                                          5-octoxy-2-(4-octoxyphenyl)pyrimidine                                                                     11 mol-%                                          5-octoxy-2-(4-hexoxyphenyl)pyrimidine                                                                     24 mol-%                                          5-octoxy-2-(4-butoxyphenyl)pyrimidine                                                                     25 mol-%                                           ##STR46##                   9 mol-%                                          4-hexoxyphenyl 4'-decoxybenzoate                                              (= compound of the general formula VII)                                        ##STR47##                  11 mol-%                                          4-hexoxyphenyl 4-octoxybenzoate                                               (compound of the general formula VII)                                         has the following liquid-crystalline ranges:                                  X 6 S.sub.c 72.3 S.sub.A 85.2 N 92 I                                          ______________________________________                                    

b) A four-component mixture which differs from the liquid-crystalmixture 16a only in that it contains no compound of the general formula(VII) has the following liquid-crystalline phase ranges:

    ______________________________________                                        X 13      S.sub.c 81.5                                                                             S.sub.A 95.5                                                                            N 98    I                                      ______________________________________                                    

Compared with the mixture 16b, the mixture 16a according to theinvention has a melting point which is lower by 7° C. and a upper S_(c)phase-range limiting temperature which is lower by 9° C.

Example 17

A ferroelectric mixture comprising the following components

    __________________________________________________________________________    mixture from Example 3a          82 mol-%                                      ##STR48##                       3 mol-%                                      Racemate of 4-(2-nonoxypyrimidin-5-yl)phenyl                                  2-chloro-4-methylpentanoate                                                    ##STR49##                       9 mol-%                                      4-pentylphenyl 4'-hexoxybenzoate                                              (compound of the general formula VIII)                                         ##STR50##                                                                    4-(2-nonoxypyrimidin-5-yl)phenyl (2S,3S)-                                                                      6 mol-%                                      2-chloro-3-methylpentanoate                                                   has the following LC ranges:                                                  X -9 S.sub.c * 61 S.sub.A * 70 N* 85 I                                        __________________________________________________________________________

At 20° C., this mixture has a polarization of -8.2 nC cm⁻², a contrastof 5.7 and, in a switching field of 10 Vμm⁻¹, a switching time of 54 μs.Comparison with the mixture from Example 12b confirms that the additionof the ester of the general formula (VIII) reduces the melting point by4° C. and increases the contrast by 24%.

Example 18

a) A liquid-crystalline mixture comprising the following nine components

    ______________________________________                                        5-octoxy-2-(4-hexoxyphenyl)pyrimidine                                                                    14 mol-%                                           5-octoxy-2-(4-octoxyphenyl)pyrimidine                                                                     5 mol-%                                           5-octoxy-2-(4-butoxyphenyl)pyrimidine                                                                    15 mol-%                                           5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                     8 mol-%                                           5-octoxy-2-(4-dodecoxyphenyl)pyrimidine                                                                   8 mol-%                                           5-octyl-2-(4-dodecoxyphenyl)pyrimidine                                                                   13 mol-%                                           [4-(5-decylpyrimidin-2-yl)]phenyl                                                                        17 mol-%                                           trans-4-pentylcyclohexanecarboxylate                                           ##STR51##                 13 mol-%                                           4-(5-octylpyrimidin-2-yl)phenyl heptanoate                                    (= compound of the general formula V)                                          ##STR52##                  7 mol-%                                           4-(10-decylpyrimidin-2-yl)phenyl undecanoate                                  (compound of the general formula V)                                           exhibits the following liquid-crystalline phase ranges:                       X -3 S.sub.c 73 S.sub.A 81 N 94 I                                             ______________________________________                                    

b) A comparable mixture which differs from the mixture 18A only in thatit contains no component of the general formula (V) has the followingliquid-crystalline phase ranges:

    ______________________________________                                        X -1       S.sub.c 81                                                                              S.sub.A 87                                                                              N 103   I                                      ______________________________________                                    

The mixture according to the invention thus has a melting point which islower by 2° C. and an upper S_(c) phase-range limiting temperature whichis lower by 8° C.

Example 19

A ferroelectric liquid-crystalline mixture comprises the following tencomponents

    ______________________________________                                        5-octoxy-2-(4-butoxyphenyl)pyrimidine                                                                    13.2 mol %                                         5-octoxy-2-(4-hexoxyphenyl)pyrimidine                                                                    14 mol %                                           5-octoxy-2-(4-octoxyphenyl)pyrimidine                                                                    5.5 mol %                                          5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                    7 mol %                                            5-octoxy-2-(4-dodecoxyphenyl)pyrimidine                                                                  7 mol %                                            [4-(5-decylpyrimidin-2-yl)]phenyl trans-                                                                 16.7 mol %                                         4-pentylcyclohexanecarboxylate                                                5-octyl-2-(4-dodecoxyphenyl)pyrimidine                                                                   11.4 mol %                                         [4-(5'-octylpyrimidin-2-yl)]phenyl heptanoate                                                            13.2 mol %                                         4-(2-nonoxypyrimidin-5-yl)phenyl 2-fluoro-                                                               6 mol %                                            4-methylpentanoate                                                            (compound of the general formula IX)                                          4-(2-nonoxypyrimidin-5-yl)phenyl (2S,3S)-                                                                6 mol %                                            2-chloro-3-methylpentanoate                                                   exhibits the following liquid-crystalline phase ranges:                       X -8     S.sub.c * 73                                                                             S.sub.A * 80                                                                             N* 90   I                                      ______________________________________                                    

and has, at 25° C., a spontaneous polarization of 9.4 nC cm⁻², acontrast of 5.8 and a switching time of 65 μs.

By comparison, the claimed ferroelectric FLC mixture from Example 7a,which differs from the abovementioned ferroelectric mixture only in thatit contains no racemate (IX), has the following phase ranges:

    ______________________________________                                        X -1       S.sub.c * 80                                                                            S.sub.A * 85                                                                            N* 99   I                                      ______________________________________                                    

In comparison, the abovementioned ferroelectric mixture has a meltingpoint which is lower by 7° C.

Example 20

a) A liquid-crystalline mixture comprising the following six components

    ______________________________________                                        5-oxtoxy-2-(4-hexoxyphenyl)pyrimidine                                                                    20.2 mol-%                                         5-octoxy-2-(4-octoxyphenyl)pyrimidine                                                                     9.4 mol-%                                         5-octoxy-2-(4-butoxyphenyl)pyrimidine                                                                    21.4 mol-%                                         5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                      17 mol-%                                         [4-(5-decylpyrimidin-2-yl)]phenyl                                                                          12 mol-%                                         trans-4-pentylcyclohexanecarboxylate                                           ##STR53##                   20 mol-%                                         [4-(5'-octoxypyrimidin-2-yl)]phenyl decanoate                                 (= compound of the general formula VI)                                        exhibits the following liquid-crystalline phase ranges:                       X 8 S.sub.c 80 S.sub.A 92 N 100 I                                             ______________________________________                                    

b) The following mixture, which is claimed in DE 3,831,2265.3 anddiffers from the mixture 20a only in that it contains no component ofthe general formula (VI), is suitable for comparison of the physicalproperties. This mixture comprising five components has the followingliquid-crystalline phase ranges:

    ______________________________________                                        X 10       S.sub.c 84                                                                              S.sub.A 93                                                                              N 105   I                                      ______________________________________                                    

In comparison with the mixture lb, the mixture 20a according to theinvention has a melting point (X) which is lower by 2° C. and an upperS_(c) phase-range limiting temperature which is lower by 4° C.

We claim:
 1. A liquid-crystalline mixture containing, as component A, atleast two 5-alkoxy-2-(alkoxyphenyl)pyrimidines of the general formula(I) ##STR54## in which the alkyl radicals are straight-chain alkylradicals in which n is an integer from 6 to 14 and x is an integer from2 to 14,wherein the liquid-crystal mixture furthermore contains at leastone component c of the general formula (V), (VI), (VII), (VIII) or theracemic mixture of one or more carboxylates of the general formula (IX)as a mixture component: ##STR55## where k may be an integer from 6 to14, and l may be an integer from 2 to 14, ##STR56## where m may be aninteger from 5 to 14, and l may be an integer from 2 to 14, ##STR57##where p may be an integer from 7 to 14, and r may be an integer from 4to 14, ##STR58## where s may be an integer from 6 to 14, and t may be aninteger from 6 to 14, ##STR59## where R² is straight-chain or branched(C₁ -C₁₂)alkyl or alkenyl, it being possible for one or two non-adjacentCH₂ groups to be replaced by O and/or S atoms, y is F, Cl, Br, CN or CF₃and R¹ is branched (C₃ -C₉)alkyl, benzyl or phenyl.
 2. Aliquid-crystalline mixture as claimed in claim 1, which contains, ascomponent C, one or more 4-(5-alkylpyrimidin-2-yl)phenyl carboxylates ofthe general formula (V) ##STR60## where k may be an integer from 6 to14, andl may be an integer from 4 to
 14. 3. A liquid-crystalline mixtureas claimed in claim 1, which contains, as component C, one or more4-(5-alkoxypyrimidin-2-yl)phenyl carboxylates of the general structure(VI) ##STR61## where m may be an integer from 6 to 14, andl may be aninteger from 4 to
 14. 4. A liquid-crystalline mixture as claimed inclaim 1, which contains, as component C, one or more benzoates of thegeneral structure (VII) ##STR62## where p may be an integer from 7 to14, andr may be an integer from 4 to
 14. 5. A liquid-crystalline mixtureas claimed in claim 1, which contains, as component C, one or morebenzoates of the general structure (VIII) ##STR63## where s may be aninteger from 6 to 14, andt may be an integer from 6 to
 14. 6. Aliquid-crystalline mixture as claimed in claim 1, which contains, ascomponent C, the racemic mixture of one or more of the followingcarboxylates of the general structure (IX) ##STR64## where R² isstraight-chain or branched (C₁ -C₁₂)alkyl or alkenyl, it being possiblefor one or two nonadjacent CH₂ groups to be replaced by O and/or Satoms,y is F or Cl and R¹ is branched (C₃ -C₉)alkyl, benzyl or phenyl.7. A liquid-crystalline mixture as claimed in claim 1, which contains,as component C, the racemic mixture of one or more of the followingcarboxylates of the general structure (IXa) ##STR65## where R² isstraight-chain or branched (C₁ -C₁₂)alkyl or alkenyl, it being possiblefor one or two nonadjacent CH₂ groups to be replaced by O and/or Satoms, andy is F or Cl.
 8. A liquid-crystalline mixture as claimed inclaim 1, which contains, as component C, the racemate of the followingester (IXb) ##STR66##
 9. A liquid-crystalline mixture as claimed inclaim 1, which contains, as component C, the racemate of the followingester (IXc) ##STR67##
 10. A liquid-crystalline mixture as claimed in anyone of claims 6 to 9, which contains, as component C, 2 to 20 mol-% ofone or more of the racemic α-substituted carboxylates.
 11. Aliquid-crystalline mixture as claimed in claim 1, which contains, ascomponent C, 2 to 40 mol-% of one or more of the mixture components ofthe general formula (V) and/or (VI) and/or (VII) and/or (VIII).
 12. Aliquid-crystalline mixture as claimed in claim 1, which further containsone or more cyclohexanecarboxylates of the general formula (II)##STR68## in which R¹ is an alkyl radical having 10 to 16 carbon atomsor an alkoxy radical having 8 to 14 carbon atoms, and R² is an alkylradical having 2 to 9 carbon atoms.
 13. A liquid-crystalline mixture asclaimed in claim 1, which further contains one or morealkenyloxyphenylpyrimidine derivatives of the general formula (III)##STR69## in which R³ is an alkyl chain having 7 to 16 carbon atoms oran alkoxy chain having 6 to 14 carbon atoms, and y is an integer from 4to
 14. 14. A liquid-crystalline mixture as claimed in claim 1, whichfurther contains one or more alkylpyrimidinealkoxyphenyl derivatives ofthe general formula (IV) ##STR70## in which m and p, independently ofone another, are integers from 6 to
 14. 15. A liquid-crystalline mixtureas claimed in claim 1, which further contains as component B (forferroelectric liquid-crystal mixtures), at least one optically activecompound from the groupa) optically active esters made fromα-chlorocarboxylic acids and mesogenic phenols, b) optically activeesters of N-acylpropline, c) optically active esters of1,3-dioxolane-4-carboxylic acids, d) optically active esters ofoxirane-2-carboxylic acids.